I also think the seperation of art and source code does not make sense (as if the art would be an unimportant, disposable component) for many games when there is a strong coupling between both. If you remove the art from a game, it no longer works, so it is an essential part of the game (and, in extension, the software itself since it has a hard dependency on the art files).
If you just look at the code, then you are not talking about the complete game anymore.
So, to recap:
To make a video game, you need code AND art AND related files (e.g. levels, data files, etc.). Removing anything of this will likely result in something that is no longer a playable game.
If code and art and related files are free-as-in-freedom, then the game is obviously free software.
If code is free but art isn't, I'd argue the game is not free software because an essential part of it is not free. It would be honest to call just the code “free” and consider an entry in the “Engines” section of the LGDB (if the code is actually useful as an engine, of course).

the cc-by-nc-sa is unlike the cc-by-sa not compatible with that definition since the it breaks rule #1: Free Distribution

the Free Software Foundation also uses the term free-software which mainly means the source code is under a FSF approved/GNU compatible license, they mostly give a sh*t about art - which is really sad ;(

But since artwork is probably the most important part of a game(without its just an engine) and because we had to choose one of them, the term Open is used and explained here:

Wuzzy, you are incorrect. Art and media can be under any license you want as they are not software. As long as the code is free software, this project should be classified as such and is incorrectly labeled.

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